Synchronous Frequency Hopping Spread Spectrum Communications

ABSTRACT

Methods, systems and metering devices are provided for performing synchronous communication in a frequency hopping communication system without the use of expensive clock circuitry and/or significant amounts of data traffic on the communication system.

FIELD OF THE INVENTION

The present invention relates to communications systems and method, and more particularly, to systems, methods and apparatus for improved performance in a wireless communications system, including without limitation a controlled mesh network that utilizes frequency hopping spread spectrum communications.

BACKGROUND

Many utilities have begun to implement frequency hopping spread spectrum (FHSS) communications in a mesh network of metering devices. Some FHSS implementations operate in a synchronous mode in which receiving devices are substantially continuously synchronized with transmitting devices in time or frequency. Such synchronous implementations typically require the use of expensive clock circuitry and produce significant amounts of data traffic as a result of the overhead required to maintain time within the system. Other FHSS implementations operate in a self-synchronous mode in which receiving devices are not at all times synchronized with transmitting devices in time or frequency. In such self-synchronous implementations, a receiving device scans a number of frequencies in the hop sequence in order to detect a preamble signal on one of the frequencies. Once detected, the receiving device will attempt to lock onto that channel, and the two devices will then begin hopping in sequence. While such self-synchronous systems do not require expensive clock circuitry and avoid the overhead to require substantially constant synchronization, such systems do typically require a long preamble to be transmitted in order to provide sufficient opportunity for a receiver to scan and lock onto a channel. This itself may result in significant data overhead.

SUMMARY OF THE INVENTION

Systems, methods and apparatus are disclosed for use in a frequency hopping communication system, in which a transmitter and a receiver hop through a sequence of predetermined communication frequencies, and in which the receiver and the transmitter may communicate on each frequency in the sequence for a predetermined period of time before hopping to a next frequency in the sequence. In one embodiment, the receiver may scan a frequency and a next frequency in a hopping sequence for a carrier signal transmitted by the transmitter. The receiver may scan both the frequency and the next frequency for a predetermined amount of time before, and a predetermined amount of time after, each expected hop from the frequency to the next frequency in the sequence. The receiver may detect a carrier signal transmitted on the frequency or the next frequency as a result of the scanning and may then lock onto the carrier signal. The receiver may then continue to hop in sequence with the transmitter. By reducing the number of scanned channels in this manner, the required preamble length may be reduced by an amount proportional to the reduction in scanned channels. A shorter preamble length may improve packet efficiency.

Other features of the systems, methods and apparatus described herein will become evident hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the systems, methods and apparatus described herein, there is shown in the drawings exemplary embodiments; however, the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings:

FIG. 1 is a diagram of an exemplary advanced metering infrastructure (AMI) system employing wireless networking;

FIG. 2 expands upon the diagram of FIG. 1 and illustrates the exemplary metering system in greater detail;

FIG. 3A is a block diagram illustrating an exemplary gatekeeper (also referred to as a “collector”) of the AMI system of FIG. 1;

FIG. 3B is a block diagram illustrating an exemplary metering device of the AMI system of FIG. 1;

FIG. 3C illustrates one embodiment of an outbound data packet format of the AMI system illustrated in FIGS. 1, 2, 3A and 3B, and FIG. 3D illustrates one embodiment of an inbound data packet format;

FIG. 4 a illustrates one embodiment of a scanning sequence performed by a receiver communicating in a synchronous mode on a communication system;

FIG. 4 b illustrates another embodiment of a scanning sequence performed by a receiver communicating in a synchronous mode on a communication system;

FIG. 5 illustrates one embodiment of a collector and a meter communicating in a synchronous mode as described herein; and

FIG. 6 illustrates one embodiment of internal synchronization amongst devices on the communication system.

DETAILED DESCRIPTION

The systems, methods and apparatus described herein may provide for synchronous communications in an AMI system that employs controlled mesh network communications. Exemplary embodiments of these systems, methods and apparatus are provided below, but it is understood that the invention is not limited to those specific embodiments. While certain details have been provided to illustrate the embodiments described below, it is understood that the invention may be practiced without those specific details. Acronyms and other terms may be used in the following description, however they are not intended to limit the scope of the invention as defined by the appended claims.

Exemplary Advanced Metering Infrastructure (Ami) System

One example of an advanced metering infrastructure (AMI) system 110 in which the systems, methods and apparatus described herein may be employed is illustrated in FIGS. 1, 2, and 3A-D. The description given herein with respect to those figures is for exemplary purposes only and is not intended in any way to limit the scope of potential embodiments.

System 110 comprises a plurality of metering devices, or “meters” 114, which are operable to sense and record consumption or usage of a service or commodity such as, for example, electricity, water, or gas. Meters 114 may be located at customer premises such as, for example, a home or place of business. Meters 114 comprise circuitry for measuring the consumption of the service or commodity being consumed at their respective locations and for generating data reflecting the consumption, as well as other data related thereto. Meters 114 may also comprise circuitry for wirelessly transmitting data generated by the meter to a remote location. Meters 114 may further comprise circuitry for receiving data, commands or instructions wirelessly as well. Meters that are operable to both receive and transmit data may be referred to as “bi-directional” or “two-way” meters (or nodes), while meters that are only capable of transmitting data may be referred to as “transmit-only” or “one-way” meters. In bi-directional meters, the circuitry for transmitting and receiving may comprise a transceiver. In an illustrative embodiment, meters 114 may be, for example, electricity meters manufactured by Elster Solutions, LLC and marketed under the trade name REX.

System 110 further comprises collectors 116. In one embodiment, collectors 116 are also meters operable to detect and record usage of a service or commodity such as, for example, electricity, water, or gas. In addition, collectors 116 are operable to send data to and receive data from meters 114. Thus, like the meters 114, the collectors 116 may comprise both circuitry for measuring the consumption of a service or commodity and for generating data reflecting the consumption and circuitry for transmitting and receiving data. In one embodiment, collector 116 and meters 114 communicate with and amongst one another using any one of several wireless techniques such as, for example, frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS). Collectors 116 are also sometimes referred to as “gatekeepers.”

A collector 116 and the meters 114 with which it communicates define a subnet or local area network (LAN) 120 of system 110. In one embodiment, each subnet or LAN may define a controlled, wireless mesh network with the collector 116 (gatekeeper) of that LAN effectively controlling the mesh network. Further details of how such a LAN is initialized, defined and maintained are described hereinafter.

As used herein, a collector 116 and the meters 114 with which it communicates may be referred to as “nodes” in the subnet/LAN 120. In each subnet/LAN 120, each meter transmits data related to consumption of the commodity being metered at the meter's location. The collector 116 receives the data transmitted by each meter 114, effectively “collecting” it, and then periodically transmits the data from all of the meters in the subnet/LAN 120 to a data collection server 206. The data collection server 206 stores the data for analysis and preparation of bills, for example. The data collection server 206 may be a specially programmed general purpose computing system and may communicate with collectors 116 via a network 112. The network 112 may comprise any form of network, including a wireless network or a fixed-wire network, such as a local area network (LAN), a wide area network (WAN), the Internet, an intranet, a telephone network, such as the public switched telephone network (PSTN), a Frequency Hopping Spread Spectrum (FHSS) radio network, a mesh network, a Wi-Fi (802.11) network, a Wi-Max (802.16) network, a land line (POTS) network, a TCP/IP network, a W-WAN, a GPRS network, a CDMA network, a Fiber network, or any combination of the above.

Referring now to FIG. 2, further details of the example AMI system 110 are shown. Typically, the system will be operated by a utility company or a company providing information technology services to a utility company. In FIG. 2, some or all of the components illustrated in dashed-box 200 may be referred to a utility's “operations center,” “head-end” or similar name. As shown, the operations center 200 may comprise a network management server 202, a network management system (NMS) 204 and the data collection server 206 that together manage one or more subnets/LANs 120 and their constituent nodes. The NMS 204 tracks changes in network state, such as new nodes registering/unregistering with the system 110, node communication paths changing, etc. This information is collected for each subnet/LAN 120 and is detected and forwarded to the network management server 202 and data collection server 206.

Each of the meters 114 and collectors 116 is assigned an identifier (LAN ID) that uniquely identifies that meter or collector on its subnet/LAN 120. In this embodiment, communication between nodes (i.e., the collectors and meters) and the system 110 is accomplished using the LAN ID. However, it is preferable for operators of a utility to query and communicate with the nodes using their own identifiers. To this end, a marriage file 208 may be used to correlate a utility's identifier for a node (e.g., a utility serial number) with both a manufacturer serial number (i.e., a serial number assigned by the manufacturer of the meter) and the LAN ID for each node in the subnet/LAN 120. In this manner, the utility can refer to the meters and collectors by the utility's identifier, while the system can employ the LAN ID for the purpose of designating particular meters during system communications.

A device configuration database 210 stores configuration information regarding the nodes. For example, in the metering system 110, the device configuration database may include data regarding time of use (TOU) switchpoints, etc. for the meters 114 and collectors 116 communicating in the system 110. A data collection requirements database 212 contains information regarding the data to be collected on a per node basis. For example, a utility may specify that metering data such as load profile, demand, TOU, etc. is to be collected from particular meter(s) 114 a. Reports 214 containing information on the network configuration may be automatically generated or in accordance with a utility request.

The network management system (NMS) 204 maintains a database describing the current state of the global fixed network system (current network state 220) and a database describing the historical state of the system (historical network state 222). The current network state 220 contains data regarding current meter-to-collector assignments, etc. for each subnet/LAN 120. The historical network state 222 is a database from which the state of the network at a particular point in the past can be reconstructed. The NMS 204 is responsible for, amongst other things, providing reports 214 about the state of the network. The NMS 204 may be accessed via an API 220 that is exposed to a user interface 216 and a Customer Information System (CIS) 218. Other external interfaces may also be implemented. In addition, the data collection requirements stored in the database 212 may be set via the user interface 216 or CIS 218.

The data collection server 206 collects data from the nodes (e.g., collectors 116) and stores the data in a database 224. The data includes metering information, such as energy consumption, and may be used for billing purposes, etc. by a utility provider.

The network management server 202, network management system 204 and data collection server 206 communicate with the nodes in each subnet/LAN 120 via network 112.

FIG. 3A is a block diagram illustrating further details of one embodiment of a collector 116. Although certain components are designated and discussed with reference to FIG. 3A, it should be appreciated that such designations and discussion are not limiting. In fact, various other components typically found in an electronic meter may be a part of collector 116, but have not been shown in FIG. 3A for the purposes of clarity and brevity. Also, other components may be used to accomplish the operation of collector 116. The components that are shown and the functionality described for collector 116 are provided as examples, and are not meant to be exclusive of other components or other functionality.

As shown in FIG. 3A, collector 116 may comprise metering circuitry 304 that performs measurement of consumption of a service or commodity and a processor 305 that controls the overall operation of the metering functions of the collector 116. The collector 116 may further comprise a display 310 for displaying information such as measured quantities and meter status and a memory 312 for storing data. The memory 312 may have stored thereon computer-executable instructions for implementing the embodiments described herein. The computer executable instruction may be implemented on processor 305 for example. The collector 116 further comprises wireless LAN communications circuitry 306 for communicating wirelessly with the meters 114 in a subnet/LAN and a network interface 308 for communication over the network 112.

In one embodiment, the metering circuitry 304, processor 305, display 310 and memory 312 are implemented using an A3 ALPHA meter available from Elster Solutions, LLC. In that embodiment, the wireless LAN communications circuitry 306 may be implemented by a LAN Option Board (e.g., a 900 MHz two-way radio) installed within the A3 ALPHA meter, and the network interface 308 may be implemented by a WAN Option Board (e.g., a telephone modem) also installed within the A3 ALPHA meter. In this embodiment, the WAN Option Board 308 routes messages from network 112 (via interface port 302) to either the meter processor 305 or the LAN Option Board 306. LAN Option Board 306 may use a transceiver (not shown), for example a 900 MHz radio, to communicate data to meters 114. Also, LAN Option Board 306 may have sufficient memory to store data received from meters 114. This data may include, but is not limited to the following: current billing data (e.g., the present values stored and displayed by meters 114), previous billing period data, previous season data, and load profile data.

LAN Option Board 306 may be capable of synchronizing its time to a real time clock (not shown) in A3 ALPHA meter, thereby synchronizing the LAN reference time to the time in the meter. The processing necessary to carry out the communication functionality and the collection and storage of metering data of the collector 116 may be handled by the processor 305 and/or additional processors (not shown) in the LAN Option Board 306 and the WAN Option Board 308.

The responsibility of a collector 116 is wide and varied. Generally, collector 116 is responsible for managing, processing and routing data communicated between the collector and network 112 and between the collector and meters 114. Collector 116 may continually or intermittently read the current data from meters 114 and store the data in a database (not shown) in collector 116. Such current data may include but is not limited to the total kWh usage, the Time-Of-Use (TOU) kWh usage, peak kW demand, and other energy consumption measurements and status information. Collector 116 also may read and store previous billing and previous season data from meters 114 and store the data in the database in collector 116. The database may be implemented as one or more tables of data within the collector 116.

In one embodiment, the LAN Option Board 306 employs a CC1110 chip available from Texas Instruments, Inc. to implement its wireless transceiver functionality. The CC 1110 chip has a built-in Received Signal Strength Indication (RSSI) capability that provides a measurement of the power present in a received radio signal. In other embodiments, a Silicon Labs Si4461 transceiver may be employed.

FIG. 3B is a block diagram of an exemplary embodiment of a meter 114 that may operate in the system 110 of FIGS. 1 and 2. As shown, the meter 114 comprises metering circuitry 304′ for measuring the amount of a service or commodity that is consumed, a processor 305′ that controls the overall functions of the meter, a display 310′ for displaying meter data and status information, and a memory 312′ for storing data and program instructions. The memory 312′ may have stored thereon computer-executable instructions for implementing the embodiments described herein. The computer executable instruction may be implemented on processor 305′ for example. The meter 114 further comprises wireless communications circuitry 306′ for transmitting and receiving data to/from other meters 114 or a collector 116. The wireless communication circuitry 306′ may comprise, for example, the aforementioned CC1110 chip available from Texas Instruments, Inc.

Referring again to FIG. 1, in one embodiment, a collector 116 directly communicates with only a subset of the plurality of meters 114 in its particular subnet/LAN. Meters 114 with which collector 116 directly communicates may be referred to as “level one” meters 114 a. The level one meters 114 a are said to be one “hop” from the collector 116. Communications between collector 116 and meters 114 other than level one meters 114 a are relayed through the level one meters 114 a. Thus, the level one meters 114 a operate as repeaters for communications between collector 116 and meters 114 located further away in subnet 120.

Each level one meter 114 a typically will only be in range to directly communicate with only a subset of the remaining meters 114 in the subnet 120. The meters 114 with which the level one meters 114 a directly communicate may be referred to as level two meters 114 b. Level two meters 114 b are one “hop” from level one meters 114 a, and therefore two “hops” from collector 116. Level two meters 114 b operate as repeaters for communications between the level one meters 114 a and meters 114 located further away from collector 116 in the subnet 120.

While only three levels of meters are shown (collector 116, first level 114 a, second level 114 b) in FIG. 1, a subnet 120 may comprise any number of levels of meters 114. For example, a subnet 120 may comprise one level of meters but might also comprise eight, sixteen, thirty-two or even more levels of meters 114. In an embodiment, as many as 2048 or more meters may be registered with a single collector 116.

As mentioned above, in one embodiment, each meter 114 and collector 116 that is installed in the system 110 has a unique identifier (LAN ID) stored thereon that uniquely identifies the device from all other devices in the system 110. Additionally, meters 114 operating in a subnet 120 comprise information including the following: data identifying the collector with which the meter is registered; the level in the subnet at which the meter is located; the repeater meter at the prior level with which the meter communicates to send and receive data to/from the collector; an identifier indicating whether the meter is a repeater for other nodes in the subnet; and if the meter operates as a repeater, the identifier that uniquely identifies the repeater within the particular subnet, and the number of meters for which it is a repeater. In one embodiment, collectors 116 have stored thereon this same data for meters 114 that are registered therewith. Thus, collector 116 comprises data identifying the nodes registered therewith as well as data identifying the registered path by which data is communicated from the collector to each such node. Each meter 114 therefore has a designated communications path to the collector that is either a direct path (e.g., all level one nodes) or an indirect path through one or more intermediate nodes that serve as repeaters.

In one embodiment, information is transmitted in the form of packets. For most network tasks such as, for example, reading meter data, collector 116 communicates with meters 114 in the subnet 120 using point-to-point transmissions. For example, a message or instruction from collector 116 may be routed through the designated set of repeaters to the desired meter 114. Similarly, a meter 114 may communicate with collector 116 through the same set of repeaters, but in reverse.

In some instances, however, collector 116 may need to quickly communicate information to all meters 114 located in its subnet 120. Accordingly, collector 116 may issue a broadcast message that is meant to reach all nodes in the subnet 120. The broadcast message may be referred to as a “flood broadcast message.” A flood broadcast originates at collector 116 and propagates through the entire subnet 120 one level at a time. For example, collector 116 may transmit a flood broadcast to all first level meters 114 a. The first level meters 114 a that receive the message pick a random time slot and retransmit the broadcast message to second level meters 114 b. Any second level meter 114 b can accept the broadcast, thereby providing better coverage from the collector out to the end point meters. Similarly, the second level meters 114 b that receive the broadcast message pick a random time slot and communicate the broadcast message to third level meters. This process continues out until the end nodes of the subnet. Thus, a broadcast message gradually propagates outward from the collector to the nodes of the subnet 120.

The flood broadcast packet header contains information to prevent nodes from repeating the flood broadcast packet more than once per level. For example, within a flood broadcast message, a field might exist that indicates to meters/nodes which receive the message, the level of the subnet the message is located; only nodes at that particular level may re-broadcast the message to the next level. If the collector broadcasts a flood message with a level of 1, only level 1 nodes may respond. Prior to re-broadcasting the flood message, the level 1 nodes increment the field to 2 so that only level 2 nodes respond to the broadcast. Information within the flood broadcast packet header ensures that a flood broadcast will eventually die out.

Generally, a collector 116 issues a flood broadcast several times, e.g. five times, successively to increase the probability that all meters in the subnet 120 receive the broadcast. A delay is introduced before each new broadcast to allow the previous broadcast packet time to propagate through all levels of the subnet.

Meters 114 may have a clock formed therein. However, meters 114 often undergo power interruptions that can interfere with the operation of any clock therein. Accordingly, it may not be possible to rely upon the clocks internal to meters 114 to provide accurate time readings. Having the correct time may be necessary, however, when time of use metering or synchronous communications are being employed. Indeed, in an embodiment, time of use schedule data may also be comprised in the same broadcast message as the time. Accordingly, collector 116 periodically flood broadcasts the real time to meters 114 in subnet 120. Meters 114 use the time broadcasts to stay synchronized with the rest of the subnet 120. In an illustrative embodiment, collector 116 broadcasts the time every 15 minutes. The broadcasts may be made near the middle of 15 minute clock boundaries that are used in performing load profiling and time of use (TOU) schedules so as to minimize time changes near these boundaries. Maintaining time synchronization is important to the proper operation of the subnet 120. Accordingly, lower priority tasks performed by collector 116 may be delayed while the time broadcasts are performed.

In an illustrative embodiment, the flood broadcasts transmitting time data may be repeated, for example, five times, so as to increase the probability that all nodes receive the time. Furthermore, where time of use schedule data is communicated in the same transmission as the timing data, the subsequent time transmissions allow a different piece of the time of use schedule to be transmitted to the nodes.

Exception messages may be used in subnet 120 to transmit unexpected events that occur at meters 114 to collector 116. In an embodiment, the first 4 seconds of every 32-second period may be allocated as an exception window for meters 114 to transmit exception messages. Meters 114 may transmit their exception messages early enough in the exception window so the message has time to propagate to collector 116 before the end of the exception window. Collector 116 may process the exceptions after the 4-second exception window. Generally, a collector 116 acknowledges exception messages, and the collector 116 waits until the end of the exception window to send this acknowledgement.

In an illustrative embodiment, exception messages may be configured as one of three different types of exception messages: local exceptions, which are handled directly by the collector 116 without intervention from data collection server 206; an immediate exception, which is generally relayed to data collection server 206 under an expedited schedule; and a daily exception, which is communicated to the data collection server 206 on a regular schedule.

Exceptions are processed as follows. When an exception is received at collector 116, the collector 116 identifies the type of exception that has been received. If a local exception has been received, collector 116 takes an action to remedy the problem. For example, when collector 116 receives an exception requesting a “node scan request” such as discussed below, collector 116 transmits a command to initiate a scan procedure to the meter 114 from which the exception was received.

If an immediate exception type has been received, collector 116 makes a record of the exception. An immediate exception might identify, for example, that there has been a power outage. Collector 116 may log the receipt of the exception in one or more tables or files. In an illustrative example, a record of receipt of an immediate exception is made in a table referred to as the “Immediate Exception Log Table.” Collector 116 then waits a set period of time before taking further action with respect to the immediate exception. For example, collector 116 may wait 64 seconds. This delay period allows the exception to be corrected before communicating the exception to the data collection server 206. For example, where a power outage was the cause of the immediate exception, collector 116 may wait a set period of time to allow for receipt of a message indicating the power outage has been corrected.

If the exception has not been corrected, collector 116 communicates the immediate exception to data collection server 206. For example, collector 116 may initiate a dial-up connection with data collection server 206 and download the exception data. After reporting an immediate exception to data collection server 206, collector 116 may delay reporting any additional immediate exceptions for a period of time such as ten minutes. This is to avoid reporting exceptions from other meters 114 that relate to, or have the same cause as, the exception that was just reported.

If a daily exception was received, the exception is recorded in a file or a database table. Generally, daily exceptions are occurrences in the subnet 120 that need to be reported to data collection server 206, but are not so urgent that they need to be communicated immediately. For example, when collector 116 registers a new meter 114 in subnet 120, collector 116 records a daily exception identifying that the registration has taken place. In an illustrative embodiment, the exception is recorded in a database table referred to as the “Daily Exception Log Table.” Collector 116 communicates the daily exceptions to data collection server 206. Generally, collector 116 communicates the daily exceptions once every 24 hours.

In one embodiment, a collector may assign designated communications paths to meters with bi-directional communication capability, and may change the communication paths for previously registered meters if conditions warrant. For example, when a collector 116 is initially brought into system 110, it needs to identify and register meters in its subnet 120. A “node scan” refers to a process of communication between a collector 116 and meters 114 whereby the collector may identify and register new nodes in a subnet 120 and allow previously registered nodes to switch paths. A collector 116 can implement a node scan on the entire subnet, referred to as a “full node scan,” or a node scan can be performed on specially identified nodes, referred to as a “single node scan.”

A full node scan may be performed, for example, when a collector is first installed. The collector 116 must identify and register nodes from which it will collect usage data. The collector 116 initiates a node scan by broadcasting a request, which may be referred to as a Node Scan Procedure request. Generally, the Node Scan Procedure request directs that all unregistered meters 114 or nodes that receive the request respond to the collector 116. The request may comprise information such as the unique address of the collector that initiated the procedure. The signal by which collector 116 transmits this request may have limited strength and therefore is detected only at meters 114 that are in proximity of collector 116. Meters 114 that receive the Node Scan Procedure request respond by transmitting their unique identifier as well as other data.

For each meter from which the collector receives a response to the Node Scan Procedure request, the collector tries to qualify the communications path to that meter before registering the meter with the collector. That is, before registering a meter, the collector 116 attempts to determine whether data communications with the meter will be sufficiently reliable. In one embodiment, the collector 116 determines whether the communication path to a responding meter is sufficiently reliable by comparing a Received Signal Strength Indication (RSSI) value (i.e., a measurement of the received radio signal strength) measured with respect to the received response from the meter to a selected threshold value. For example, the threshold value may be −60 dBm. RSSI values above this threshold would be deemed sufficiently reliable. In another embodiment, qualification is performed by transmitting a predetermined number of additional packets to the meter, such as ten packets, and counting the number of acknowledgements received back from the meter. If the number of acknowledgments received is greater than or equal to a selected threshold (e.g., 8 out of 10), then the path is considered to be reliable. In other embodiments, a combination of the two qualification techniques may be employed.

If the qualification threshold is not met, the collector 116 may add an entry for the meter to a “Straggler Table.” The entry includes the meter's LAN ID, its qualification score (e.g., 5 out of 10; or its RSSI value), its level (in this case level one) and the unique ID of its parent (in this case the collector's ID).

If the qualification threshold is met or exceeded, the collector 116 registers the node. Registering a meter 114 comprises updating a list of the registered nodes at collector 116. For example, the list may be updated to identify the meter's system-wide unique identifier and the communication path to the node. Collector 116 also records the meter's level in the subnet (i.e. whether the meter is a level one node, level two node, etc.), whether the node operates as a repeater, and if so, the number of meters for which it operates as a repeater. The registration process further comprises transmitting registration information to the meter 114. For example, collector 116 forwards to meter 114 an indication that it is registered, the unique identifier of the collector with which it is registered, the level the meter exists at in the subnet, and the unique identifier of its parent meter that will serve as a repeater for messages the meter may send to the collector. In the case of a level one node, the parent is the collector itself. The meter stores this data and begins to operate as part of the subnet by responding to commands from its collector 116.

Qualification and registration continues for each meter that responds to the collector's initial Node Scan Procedure request. The collector 116 may rebroadcast the Node Scan Procedure additional times so as to insure that all meters 114 that may receive the Node Scan Procedure have an opportunity for their response to be received and the meter qualified as a level one node at collector 116.

The node scan process then continues by performing a similar process as that described above at each of the now registered level one nodes. This process results in the identification and registration of level two nodes. After the level two nodes are identified, a similar node scan process is performed at the level two nodes to identify level three nodes, and so on.

Specifically, to identify and register meters that will become level two meters, for each level one meter, in succession, the collector 116 transmits a command to the level one meter, which may be referred to as an “Initiate Node Scan Procedure” command. This command instructs the level one meter to perform its own node scan process. The request comprises several data items that the receiving meter may use in completing the node scan. For example, the request may comprise the number of timeslots available for responding nodes, the unique address of the collector that initiated the request, and a measure of the reliability of the communications between the target node and the collector. As described below, the measure of reliability may be employed during a process for identifying more reliable paths for previously registered nodes.

The meter that receives the Initiate Node Scan Procedure request responds by performing a node scan process similar to that described above. More specifically, the meter broadcasts a request to which all unregistered nodes may respond. The request comprises the number of timeslots available for responding nodes (which is used to set the period for the node to wait for responses), the unique address of the collector that initiated the node scan procedure, a measure of the reliability of the communications between the sending node and the collector (which may be used in the process of determining whether a meter's path may be switched as described below), the level within the subnet of the node sending the request, and an RSSI threshold (which may also be used in the process of determining whether a registered meter's path may be switched). The meter issuing the node scan request then waits for and receives responses from unregistered nodes. For each response, the meter stores in memory the unique identifier of the responding meter. This information is then transmitted to the collector.

For each unregistered meter that responded to the node scan issued by the level one meter, the collector attempts again to determine the reliability of the communication path to that meter. In one embodiment, the collector sends a “Qualify Nodes Procedure” command to the level one node which instructs the level one node to transmit a predetermined number of additional packets to the potential level two node and to record the number of acknowledgements received back from the potential level two node. This qualification score (e.g., 8 out of 10) is then transmitted back to the collector, which again compares the score to a qualification threshold. In other embodiments, other measures of the communications reliability may be provided, such as an RSSI value.

If the qualification threshold is not met, then the collector adds an entry for the node in the Straggler Table, as discussed above. However, if there already is an entry in the Straggler Table for the node, the collector will update that entry only if the qualification score for this node scan procedure is better than the recorded qualification score from the prior node scan that resulted in an entry for the node.

If the qualification threshold is met or exceeded, the collector 116 registers the node. Again, registering a meter 114 at level two comprises updating a list of the registered nodes at collector 116. For example, the list may be updated to identify the meter's unique identifier and the level of the meter in the subnet. Additionally, the collector's 116 registration information is updated to reflect that the meter 114 from which the scan process was initiated is identified as a repeater (or parent) for the newly registered node. The registration process further comprises transmitting information to the newly registered meter as well as the meter that will serve as a repeater for the newly added node. For example, the node that issued the Initiate Node Scan Procedure request is updated to identify that it operates as a repeater and, if it was previously registered as a repeater, increments a data item identifying the number of nodes for which it serves as a repeater. Thereafter, collector 116 forwards to the newly registered meter an indication that it is registered, an identification of the collector 116 with which it is registered, the level the meter exists at in the subnet, and the unique identifier of the node that will serve as its parent, or repeater, when it communicates with the collector 116.

The collector then performs the same qualification procedure for each other potential level two node that responded to the level one node's node scan request. Once that process is completed for the first level one node, the collector initiates the same procedure at each other level one node until the process of qualifying and registering level two nodes has been completed at each level one node. Once the node scan procedure has been performed by each level one node, resulting in a number of level two nodes being registered with the collector, the collector will then send the Initiate Node Scan Procedure request to each level two node, in turn. Each level two node will then perform the same node scan procedure as performed by the level one nodes, potentially resulting in the registration of a number of level three nodes. The process is then performed at each successive node, until a maximum number of levels is reached (e.g., seven levels) or no unregistered nodes are left in the subnet.

It will be appreciated that in the present embodiment, during the qualification process for a given node at a given level, the collector qualifies the last “hop.” For example, if an unregistered node responds to a node scan request from a level four node, and therefore, becomes a potential level five node, the qualification score for that node is based on the reliability of communications between the level four node and the potential level five node (i.e., packets transmitted by the level four node versus acknowledgments received from the potential level five node), not based on any measure of the reliability of the communications over the full path from the collector to the potential level five node. In other embodiments, of course, the qualification score could be based on the full communication path.

At some point, each meter will have an established communication path to the collector which will be either a direct path (i.e., level one nodes) or an indirect path through one or more intermediate nodes that serve as repeaters. If during operation of the network, a meter registered in this manner fails to perform adequately, it may be assigned a different path or possibly to a different collector as described below.

As previously mentioned, a full node scan may be performed when a collector 116 is first introduced to a network. At the conclusion of the full node scan, a collector 116 will have registered a set of meters 114 with which it communicates and reads metering data. Full node scans might be periodically performed by an installed collector to identify new meters 114 that have been brought on-line since the last node scan and to allow registered meters to switch to a different path.

In addition to the full node scan, collector 116 may also perform a process of scanning specific meters 114 in the subnet 120, which is referred to as a “single node scan.” For example, collector 116 may issue a specific request to a meter 114 to perform a node scan outside of a full node scan when on a previous attempt to scan the node, the collector 116 was unable to confirm that the particular meter 114 received the node scan request. Also, a collector 116 may request a single node scan of a meter 114 when during the course of a full node scan the collector 116 was unable to read the node scan data from the meter 114. Similarly, a single node scan will be performed when an exception procedure requesting an immediate node scan is received from a meter 114.

The system 110 also automatically reconfigures to accommodate a new meter 114 that may be added. More particularly, the system identifies that the new meter has begun operating and identifies a path to a collector 116 that will become responsible for collecting the metering data. Specifically, the new meter will broadcast an indication that it is unregistered. In one embodiment, this broadcast might be, for example, embedded in, or relayed as part of a request for an update of the real time as described above. The broadcast will be received at one of the registered meters 114 in proximity to the meter that is attempting to register. The registered meter 114 forwards the time to the meter that is attempting to register. The registered node also transmits an exception request to its collector 116 requesting that the collector 116 implement a node scan, which presumably will locate and register the new meter. The collector 116 then transmits a request that the registered node perform a node scan. The registered node will perform the node scan, during which it requests that all unregistered nodes respond. Presumably, the newly added, unregistered meter will respond to the node scan. When it does, the collector will then attempt to qualify and then register the new node in the same manner as described above.

Once a communication path between the collector and a meter is established, the meter can begin transmitting its meter data to the collector and the collector can transmit data and instructions to the meter. Data transmission between a collector and the meters in its subnet are, in one embodiment, performed in accordance with the following communications protocol. In this protocol, data is transmitted in packets. “Outbound” packets are packets transmitted from the collector to a meter at a given level. In one embodiment, as illustrated in FIG. 3C, outbound packets contain the following fields, but other fields may also be included:

-   -   Length—the length of the packet;     -   SrcAddr—source address—in this case, the LAN ID of the         collector;     -   DestAddr—the LAN ID of the meter to which the packet is         addressed;     -   RptPath—the communication path to the destination meter (i.e.,         the list of identifiers of each repeater in the path from the         collector to the destination node); and     -   Data—the payload of the packet.         The packet may also include integrity check information (e.g.,         CRC), a pad to fill-out unused portions of the packet and other         control information. When the packet is transmitted from the         collector, it will only be forwarded on to the destination meter         by those repeater meters whose identifiers appear in the RptPath         field. Other meters may receive the packet, but meters that are         not listed in the path identified in the RptPath field will not         repeat the packet.

“Inbound” packets are packets transmitted from a meter at a given level to the collector. In one embodiment, as illustrated in FIG. 3D, inbound packets contain the following fields, but other fields may also be included:

-   -   Length—the length of the packet;     -   SrcAddr—source address—the LAN ID of the meter that initiated         the packet;     -   DestAddr—the LAN ID of the collector to which the packet is to         be transmitted;     -   RptAddr—an identifier of the parent node that serves as the next         repeater for the sending node;     -   Data—the payload of the packet;         Because each meter knows the identifier of its parent node         (i.e., the node in the next lower level that serves as a         repeater for the present node), an inbound packet need only         identify who is the next parent. When a node receives an inbound         packet, it checks to see if the RptAddr matches its own         identifier. If not, it discards the packet. If so, it knows that         it is supposed to forward the packet on toward the collector.         The node will then replace the RptAddr field with the identifier         of its own parent and will then transmit the packet so that its         parent will receive it. This process will continue through each         repeater at each successive level until the packet reaches the         collector.

For example, suppose a meter at level three initiates transmission of a packet destined for its collector. The level three node will insert in the RptAddr field of the inbound packet the identifier of the level two node that serves as a repeater for the level three node. The level three node will then transmit the packet. Several level two nodes may receive the packet, but only the level two node having an identifier that matches the identifier in the RptAddr field of the packet will acknowledge it. The others will discard it. When the level two node with the matching identifier receives the packet, it will replace the RptAddr field of the packet with the identifier of the level one node that serves as a repeater for that level two node, and the level two node will then transmit the packet. This time, the level one node having the identifier that matches the RptAddr field will receive the packet. The level one node will insert the identifier of the collector in the RptAddr field and will transmit the packet. The collector will then receive the packet to complete the transmission.

A collector 116 periodically retrieves meter data from the meters that are registered with it. For example, meter data may be retrieved from a meter every 4 hours. Where there is a problem with reading the meter data on the regularly scheduled interval, the collector will try to read the data again before the next regularly scheduled interval. Nevertheless, there may be instances wherein the collector 116 is unable to read metering data from a particular meter 114 for a prolonged period of time. The meters 114 store an indication of when they are read by their collector 116 and keep track of the time since their data has last been collected by the collector 116. If the length of time since the last reading exceeds a defined threshold, such as for example, 18 hours, presumably a problem has arisen in the communication path between the particular meter 114 and the collector 116. Accordingly, the meter 114 changes its status to that of an unregistered meter and attempts to locate a new path to a collector 116 via the process described above for a new node. Thus, the exemplary system is operable to reconfigure itself to address inadequacies in the system.

In some instances, while a collector 116 may be able to retrieve data from a registered meter 114 occasionally, the level of success in reading the meter may be inadequate. For example, if a collector 116 attempts to read meter data from a meter 114 every 4 hours but is able to read the data, for example, only 70 percent of the time or less, it may be desirable to find a more reliable path for reading the data from that particular meter. Where the frequency of reading data from a meter 114 falls below a desired success level, the collector 116 transmits a message to the meter 114 to respond to node scans going forward. The meter 114 remains registered but will respond to node scans in the same manner as an unregistered node as described above. In other embodiments, all registered meters may be permitted to respond to node scans, but a meter will only respond to a node scan if the path to the collector through the meter that issued the node scan is shorter (i.e., less hops) than the meter's current path to the collector. A lesser number of hops is assumed to provide a more reliable communication path than a longer path. A node scan request always identifies the level of the node that transmits the request, and using that information, an already registered node that is permitted to respond to node scans can determine if a potential new path to the collector through the node that issued the node scan is shorter than the node's current path to the collector.

If an already registered meter 114 responds to a node scan procedure, the collector 116 recognizes the response as originating from a registered meter but that by re-registering the meter with the node that issued the node scan, the collector may be able to switch the meter to a new, more reliable path. The collector 116 may verify that the RSSI value of the node scan response exceeds an established threshold. If it does not, the potential new path will be rejected. However, if the RSSI threshold is met, the collector 116 will request that the node that issued the node scan perform the qualification process described above (i.e., send a predetermined number of packets to the node and count the number of acknowledgements received). If the resulting qualification score satisfies a threshold, then the collector will register the node with the new path. The registration process comprises updating the collector 116 and meter 114 with data identifying the new repeater (i.e. the node that issued the node scan) with which the updated node will now communicate. Additionally, if the repeater has not previously performed the operation of a repeater, the repeater would need to be updated to identify that it is a repeater. Likewise, the repeater with which the meter previously communicated is updated to identify that it is no longer a repeater for the particular meter 114. In other embodiments, the threshold determination with respect to the RSSI value may be omitted. In such embodiments, only the qualification of the last “hop” (i.e., sending a predetermined number of packets to the node and counting the number of acknowledgements received) will be performed to determine whether to accept or reject the new path.

In some instances, a more reliable communication path for a meter may exist through a collector other than that with which the meter is registered. A meter may automatically recognize the existence of the more reliable communication path, switch collectors, and notify the previous collector that the change has taken place. The process of switching the registration of a meter from a first collector to a second collector begins when a registered meter 114 receives a node scan request from a collector 116 other than the one with which the meter is presently registered. Typically, a registered meter 114 does not respond to node scan requests. However, if the request is likely to result in a more reliable transmission path, even a registered meter may respond. Accordingly, the meter determines if the new collector offers a potentially more reliable transmission path. For example, the meter 114 may determine if the path to the potential new collector 116 comprises fewer hops than the path to the collector with which the meter is registered. If not, the path may not be more reliable and the meter 114 will not respond to the node scan. The meter 114 might also determine if the RSSI of the node scan packet exceeds an RSSI threshold identified in the node scan information. If so, the new collector may offer a more reliable transmission path for meter data. If not, the transmission path may not be acceptable and the meter may not respond. Additionally, if the reliability of communication between the potential new collector and the repeater that would service the meter meets a threshold established when the repeater was registered with its existing collector, the communication path to the new collector may be more reliable. If the reliability does not exceed this threshold, however, the meter 114 does not respond to the node scan.

If it is determined that the path to the new collector may be better than the path to its existing collector, the meter 114 responds to the node scan. Included in the response is information regarding any nodes for which the particular meter may operate as a repeater. For example, the response might identify the number of nodes for which the meter serves as a repeater.

The collector 116 then determines if it has the capacity to service the meter and any meters for which it operates as a repeater. If not, the collector 116 does not respond to the meter that is attempting to change collectors. If, however, the collector 116 determines that it has capacity to service the meter 114, the collector 116 stores registration information about the meter 114. The collector 116 then transmits a registration command to meter 114. The meter 114 updates its registration data to identify that it is now registered with the new collector. The collector 116 then communicates instructions to the meter 114 to initiate a node scan request. Nodes that are unregistered, or that had previously used meter 114 as a repeater respond to the request to identify themselves to collector 116. The collector registers these nodes as is described above in connection with registering new meters/nodes.

Under some circumstances it may be necessary to change a collector. For example, a collector may be malfunctioning and need to be taken off-line. Accordingly, a new communication path must be provided for collecting meter data from the meters serviced by the particular collector. The process of replacing a collector is performed by broadcasting a message to unregister, usually from a replacement collector, to all of the meters that are registered with the collector that is being removed from service. In one embodiment, registered meters may be programmed to only respond to commands from the collector with which they are registered. Accordingly, the command to unregister may comprise the unique identifier of the collector that is being replaced. In response to the command to unregister, the meters begin to operate as unregistered meters and respond to node scan requests. To allow the command to unregister to propagate through the subnet, when a node receives the command it will not unregister immediately, but rather remain registered for a defined period, which may be referred to as the “Time to Live”. During this time to live period, the nodes continue to respond to application layer and immediate retries allowing the unregister command to propagate to all nodes in the subnet. Ultimately, the meters register with the replacement collector using the procedure described above.

One of the collector's 116 main responsibilities within subnet 120 is to retrieve metering data from meters 114. In one embodiment, collector 116 has as a goal to obtain at least one successful read of the metering data per day from each node in its subnet. Collector 116 attempts to retrieve the data from all nodes in its subnet 120 at a configurable periodicity. For example, collector 116 may be configured to attempt to retrieve metering data from meters 114 in its subnet 120 once every 4 hours. In greater detail, in one embodiment, the data collection process begins with the collector 116 identifying one of the meters 114 in its subnet 120. For example, collector 116 may review a list of registered nodes and identify one for reading. The collector 116 then communicates a command to the particular meter 114 that it forward its metering data to the collector 116. If the meter reading is successful and the data is received at collector 116, the collector 116 determines if there are other meters that have not been read during the present reading session. If so, processing continues. However, if all of the meters 114 in subnet 120 have been read, the collector waits a defined length of time, such as, for example, 4 hours, before attempting another read.

If during a read of a particular meter, the meter data is not received at collector 116, the collector 116 begins a retry procedure wherein it attempts to retry the data read from the particular meter. Collector 116 continues to attempt to read the data from the node until either the data is read or the next subnet reading takes place. In an embodiment, collector 116 attempts to read the data every 60 minutes. Thus, wherein a subnet reading is taken every 4 hours, collector 116 may issue three retries between subnet readings.

In one embodiment, data collected and stored in the meters 114 of the system 110 of FIGS. 1, 2, 3A and 3B is organized and extracted from each meter 114 in accordance with American National Standards Institute (ANSI) standard C12.19. The ANSI C12.19 standard defines a table structure for utility application data to be passed between an end device, such as a meter 114, and a computer, such as the Network Management Server 204 of FIG. 2. The purpose of the tables is to define structures for transporting data to and from end devices. C12.19 defines both a “standard table” structure and a “manufacturers table” structure. In this embodiment, the Network Management Server 204 includes a set of commands for reading data from, and writing data to, one or more C12.19 tables in an end device, such as a meter 114. Those commands may be transmitted to a meter 114 or other node in accordance with the wireless networking protocol described above.

Systems, Methods and Apparatus for Communication Synchronization

Described below are various techniques (methods, systems and/or apparatus) for performing synchronous communications in a system which employs wireless communications, including but not limited to controlled, wireless mesh network communications.

In one embodiment, in a frequency hopping communication system, in which a transmitter and a receiver hop through a sequence of predetermined communication frequencies, the receiver and the transmitter may communicate on each frequency in the sequence for a predetermined period of time before hopping to a next frequency in the sequence. In order to synchronize the receiver with the transmitter, the receiver may scan a frequency and a next frequency in a hopping sequence for a carrier signal transmitted by the transmitter. The receiver may scan both frequencies for a predetermined amount of time before, and a predetermined amount of time after, each expected hop from that frequency to that next frequency in the sequence. The receiver may detect a carrier signal transmitted on the frequency or the next frequency as a result of the scanning and may then lock onto the carrier signal. The receiver may then continue to hop in sequence with the transmitter. By reducing the number of scanned channels in this manner, the required preamble length may be reduced by an amount proportional to the reduction in scanned channels. A shorter preamble length may improve packet efficiency.

In another embodiment, in addition to scanning the one frequency and the next frequency for the predetermined amount of time before and after an expected hop, an additional integer number (N) of consecutive frequencies in said sequence immediately before said one frequency and an additional N consecutive frequencies in said sequence immediately after said next frequency may be scanned, wherein N is not greater than 4.

In one embodiment, the predetermined period of time for which the receiver and transmitter communicate on a given frequency before hopping to a next frequency may comprise 400 milliseconds, the predetermined amount of time that scanning is performed before an expected hop from one frequency to the next may comprise 200 milliseconds, and the predetermined amount of time that scanning is performed after the expected hop from one frequency to the next may also comprise 200 milliseconds.

In other embodiments, a receiver may periodically receive from the transmitter a time synchronization message, and may then synchronize a clock within the receiver with a clock within the transmitter based on the time synchronization message. For example, in one embodiment, the receiver may receive a time synchronization message from the transmitter about every fifteen minutes.

In one embodiment, after the receiver has locked onto a carrier signal, the receiver may receive a packet of information from the transmitter, wherein the packet includes a value indicative of a value of a clock of the transmitter. The receiver may then adjust a value of a clock within the receiver based on the value in the received packet.

In yet other embodiments, a receiver may transmit a synch request message, and may then receive from a transmitter a time synchronization message in response to the synch request message. The receiver may then synchronize a clock within the receiver with a clock within the transmitter based on the time synchronization message.

FIGS. 4 a, 4 b, and 5 provide further details of various embodiments of the techniques described above that may be performed by a receiver to allow a receiver and a transmitter to communicate in a synchronous mode. A transmitter may be a metering device in the communication system, such as a meter 114, collector 116, or similar metering device for example. A receiver may also be a metering device, such as a meter 114, collector 116, or similar metering device for example.

As illustrated in FIGS. 4 a, 4 b, and 5, the transmitter and the receiver may communicate in a synchronous mode using a sequence of predetermined communication frequencies. Frequencies Fn−1, Fn, and Fn+1 may be sequential channels in the predetermined sequence of channels on which the transmitter and receiver may communicate. For example, the transmitter and receiver may perform frequency hopping spread spectrum (FHSS) communications in the 900 MHz ISM band and may utilize twenty-five (25) different frequency channels in a hop sequence. In such a system, the transmitter may be limited to 0.25 watts. The transmitter may transmit packets at a rate in the range of 18 kbps.

The receiver and the transmitter may be configured to communicate on each frequency in the sequence for a predetermined amount of time before hopping to a next frequency in the sequence. The receiver and transmitter may each have a respective internal clock for keeping track of each predetermined amount of time before performing the next hop in the sequence. At the end of every predetermined period of time, the receiver and transmitter may know to hop from one frequency to the next frequency in the sequence of channels based on the time according to their respective internal clocks. Alternatively, the receiver and/or transmitter may receive the time from an external source, such as a network server for example. The predetermined period of time on which the communication system operates may include any predetermined amount of time that allows the receiver and transmitter to perform the synchronous communications described herein.

While the transmitter and the receiver may communicate in a synchronous mode, the transmitter and the receiver may not be totally time and frequency synchronized. For example, the internal clock of the receiver may not be totally time synchronized with the internal clock of the transmitter. The transmitter and receiver may, however, work in a synchronous mode by implementing a time slotting mechanism using a scanning sequence in the receiving device. This may allow the transmitter and the receiver to work in a synchronous mode without the use of extremely tight time control within each device.

According to one embodiment, a receiver may perform a scanning sequence to compensate for any delay between the internal clock of the receiver and transmitter. Based on the timing of the receiver's internal clock and the sequence of predetermined communication channels, the receiver may know approximately when a transmitter may be scheduled to perform a hop from one frequency to the next in the sequence. The receiver may scan a number of channels before and after an expected hop based on which frequency the receiver believes the transmitter may be transmitting. For example, the receiver may perform a scan on frequency Fn−1 and frequency Fn if the receiver believes the transmitter is transmitting on frequency Fn−1. Such a scan may compensate for the internal clock of the receiver being behind the internal clock of the transmitter. The receiver may also perform a scan on frequency Fn−1 and frequency Fn if the receiver believes the transmitter is transmitting on frequency Fn. Such a scan may compensate for the internal clock of the receiver being ahead of the internal clock of the transmitter.

FIG. 4 a illustrates an example of a scanning sequence that may be performed by a receiver in accordance with the methods described herein. As illustrated in FIG. 4 a, the receiver and the transmitter may be configured to communicate on each frequency in a predetermined sequence of channels for a 400 ms period of time. The receiver and transmitter may be configured to hop to the next frequency in the sequence every 400 ms. The predetermined period of time for which the receiver may scan before and after each expected hop in this example is 200 ms. The 200 ms period of time before and after each 400 ms boundary may correspond to a calculation of slip time in communications between the transmitter and the receiver. For example, using a transmitter and/or receiver with 26 MHz crystals with +/−45 PPM, there may be potentially 90 PPM of error between devices. Assuming communications may be received every 15 minutes within the communication system, the maximum timing error may be approximately 80 ms at the end of any 15 minute period. If each device on the communication system changes channels every 400 ms, as illustrated in FIG. 4 a, a receiver may scan two channels when the receiver is within 80 ms of an expected hop from one channel to the next to account for any potential error between the internal clock of the transmitter and the internal clock of the receiver. Expanding the worst case tolerance to 200 ms may allow a receiver to miss one time synch message and still remain synchronized in a 30 minute period, assuming that two channels are scanned at any time. The 200 ms period of time is merely illustrative as the receiver may perform a frequency scan for any period of time before and after an expected hop from one channel to the next.

As shown in FIG. 4 a, the receiver may compensate for a calculated 200 ms delay by scanning frequencies Fn−1 and Fn for the 200 ms period of time prior to and the 200 ms period of time after the 400 ms boundary at 402. The 400 ms boundary at 402 is the boundary at which the receiver and/or transmitter may be configured to hop from frequency Fn−1 to frequency Fn. Thus, frequency Fn−1 is the channel on which the transmitter is scheduled to transmit prior to the 400 ms boundary at 402, and frequency Fn is the channel on which the transmitter is scheduled to transmit after the 400 ms boundary at 402.

The receiver will continue to scan on frequencies FN−1 and Fn throughout the 400 msec window (200 msec before and 200 msec after the 400 ms boundary), until the next boundary at which the hop frequency is to be incremented. During the scan, the receiver may detect a communication transmitted by the transmitter on frequency Fn−1 or frequency Fn. For example, the receiver may detect a carrier signal or the like being transmitted by the transmitter. If the receiver detects a carrier signal transmitted on frequency Fn−1 or frequency Fn as a result of the scan, the receiver may lock on to the carrier signal and continue to hop in sequence with the transmitter until a valid message is completely received. After a valid message is complete, the receiver returns to scanning. If the receiver does not detect a carrier signal transmitted by the transmitter on frequency Fn−1 or frequency Fn during the 400 ms period, then the receiver will start scaning the next set of frequencies (e.g., Fn and Fn+1) beginning 200 msec before the next 400 ms boundary and continuing until 200 msec after it unless, again, a valid message is detected and received. Generally, in this embodiment, the receiver will be scanning any time it is not in the process of receiving a valid message.

In the present embodiment, the receiver works through the potential channels and looks for the channel with the highest RSSI. It takes that signal and evaluates enough samples to determine if a pre-determined preamble bit pattern is present. If so, it will stay on that channel and continue to follow the preamble bits until it receives a start frame delimiter (SFD). If the signal does not have a bit pattern that matches the preamble, the receiver will move to the next highest RSSI channel and follow the same preamble check. If none of the potential channels fits, the receiver will go back and again scan for RSSI, and repeat the procedure. When it receives a preamble and SFD, it will lock onto that channel and continue to receive the complete message.

By reducing the number of scanned channels in this manner, the required preamble length may be reduced by an amount proportional to the reduction in scanned channels. A shorter preamble length may improve packet efficiency. For example, in a conventional FHSS system, a preamble time of 65 msec may be required to enable receivers to lock onto a given channel. By implementing the scanning techniques described herein, the required preamble time may be reduced to as low as 18.7 msec.

While the scanning sequence above is described by illustrating a scan of two frequencies at a time, the scanning sequence is not limited to scanning two frequencies. In other embodiments, a receiver may scan an additional integer number (N) of consecutive frequencies in the hop sequence immediately before the one frequency (e.g., Fn−1) and an additional N consecutive frequencies in the sequence immediately after the next frequency (e.g., Fn). Preferably, however, N is not greater than 4. For example, as illustrated in FIG. 4 b, a receiver may scan frequencies Fn−1, Fn, and Fn+1 for a 200 ms period of time before and after an expected hop at the 400 ms boundary at 402′. The receiver may perform such a scan if, based on the receiver's internal clock, the receiver determines that a transmitter is transmitting on frequency Fn for example. Performing such a scan may allow the receiver to compensate for the internal clock of the receiver being ahead of or behind the internal clock of the transmitter.

FIG. 5 illustrates a block diagram of a collector and a meter communicating in a synchronous mode as described herein. Collector 116 may be a transmitter and/or a receiver, and may communicate with a meter, another collector, or similar metering device. Meter 114 may also be a transmitter and/or a receiver, and may communicate with a collector, another meter, or similar metering device. As shown in FIG. 5, collector 116 is a transmitter that is communicating with meter 114, which is a receiver.

Collector 116 and meter 114 may communicate in a synchronous mode by hopping through predetermined communication frequencies Fn−1, Fn, and Fn+1. For example, collector 116 and meter 114 may communicate on frequency Fn−1 at 506 a for a predetermined amount of time before hopping to frequency Fn. Collector 116 and meter 114 may then communicate on frequency Fn at 506 b for a predetermined amount of time before hopping to frequency Fn+1. Collector 116 and meter 114 may continue to communicate by hopping from one channel to the next in the sequence as described above.

The collector 116 and meter 114 may know when to perform a hop from one frequency to the next based on the time according to an internal clock in each device. Collector 116 and meter 114 each have a respective clock 502 and clock 504 for keeping track of the time on which the communication system is operating. Clock 502 and clock 504 may each keep track of the time by using the relative time since midnight, for example. Collector 116 may know when to hop to another channel based on the time according to clock 502. Similarly, meter 114 may know when to hop to another channel based on the time according to clock 504. Meter 114 may also know approximately when collector 116 is going to hop to another channel based on the time according to clock 504. As the time according to clock 504 may not be completely in synch with clock 502, meter 114 may synchronize with collector 116 by scanning any number of channels before and/or after an expected hop by collector 116 according to clock 504.

For example, the meter 114 may determine, based on the time according to clock 504, that collector 116 may be transmitting on frequency Fn at 506 b. As a result, meter 114 may scan frequencies Fn−1 at 506 a and Fn at 506 b. Such a scan may compensate for any latency in communication due to clock 504 being ahead of clock 502. Alternatively, meter 114 may scan frequencies Fn at 506 b and Fn+1 at 506 c. Such a scan may compensate for any latency in the communication system due to clock 504 being behind clock 502. Meter 114 may even scan frequencies Fn−1 at 506 a, Fn at 506 b, and Fn+1 at 506 c, or any other combination of frequencies, to compensate for any latency in the communication system due to clock 504 an clock 502 not being in synchronization with one another. The frequency scan may be performed as described above with regard to the description of FIGS. 4 a and 4 b.

Meter 114 may detect a carrier signal transmitted by collector 116 as a result of performing a frequency scan and may lock onto the carrier signal. Once the meter 114 locks on to the carrier signal, the meter 114 may receive a packet of information from the collector 116. In one embodiment, the packet of information may include a value indicative of a value of a clock of the transmitter. After meter 114 has locked on to the carrier signal, meter 114 may continue to hop in sequence with collector 116.

FIG. 6 is a block diagram illustrating an internal synchronization amongst a receiver and a transmitter in a communication system. As described above, each device on the system may keep its own time internally. A transmitter and/or receiver in the communication system may regain and/or maintain synchronization through reception of an outbound communication on the system. A receiver may receive an indication of time in a communication and may set its internal clock to the time according to a received communication.

According to one embodiment, as illustrated in FIG. 6, collector 116, meter 114 a, and/or meter 114 b may maintain synchronization through communication of outbound packets. Collector 116, meter 114 a, and/or meter 114 b may be a transmitter and/or receiver in a communication system as described herein. Collector 116, meter 114 a, and/or meter 114 b may be a transmitter if they are transmitting an outbound communication and may be a receiver if they are receiving an outbound communication. The outbound packets may be transmitted and received by the receiver once the receiver has locked on to the carrier signal of the transmitter as described above with regard to FIGS. 4 a, 4 b, and 5. While three levels of meters are shown (collector 116, first level 114 a, and second level 114 b) in FIG. 6, the meters may be part of a subnet that may comprise any number of levels of meters 114.

As illustrated in FIG. 6, collector 116 may transmit an outbound communication, such as packet A 602. Packet A 602 may include time 608 which may correspond to the time on which the system is operating. For example, the time 608 may correspond to the time according to internal clock 606 that is kept by collector 116. In one embodiment, Packet A 602 may be an outbound broadcast performed by collector 116 every 15 minutes.

Meter 114 a may receive packet A 602 and may set its internal clock 610 to correspond to the time 608 in the received packet A 602. Meter 114 a may adjust time 608 to account for processing delays and/or packet transmission delays prior to using time 608 as the new time for internal clock 610.

Meter 114 a may re-transmit the message received in packet A 602 to meter 114 b. For example, meter 114 a may operate as a repeater (or parent) for meter 114 b. Instead of just forwarding packet A 602, meter 114 a may ensure that it is transmitting the correct time by re-transmitting the message in packet B 604 with the proper timing according to internal clock 610. Meter 114 a may set the time 612 in the packet B 604 so the time is correct for the next hop. The time adjustments may account for processing delays and/or packet transmission delays so the time 612 may be correct when received and used as the new time by meter 114 b. Meter 114 b may receive packet B 604 and set its internal clock 614 according to the time 612 indicated in the received packet B 604. Meter 114 b may also adjust time 612 prior to being used as the new time for internal clock 614 to account for processing delays and/or packet transmission delays.

Any transmitter, such as collector 116, meter 114 a, and/or meter 114 b, may be prevented from transmitting a packet if the time according to the transmitter's internal clock is not valid. However, there may be a field in each packet that indicates if the time according to the internal clock is absolute or relative time. This may allow collector 116, meter 114 a, and/or meter 114 b to transmit a packet when it doesn't know the absolute time without impacting a device receiving the packet or any other device on the communication system. A device receiving a packet having a relative time may not set its internal clock according to the received time.

The time in an outbound packet, such as packet A 602 or packet B 604, may be the relative time since midnight. For example, the time in packet A 602 and/or packet B 604 may include a 3 byte quantity that provides the number of 1/128 second increments that have occurred since midnight. This may be approximately 7.8 millisecond resolution. An additional small error term may be considered due to latency within the network that may create uncertainty regarding the start of the packet. To maintain an accurate timebase across all metering devices, a transmitter that is transmitting an outbound packet may set the packet's time field equal to the moment that a first bit of a Start Frame Delimiter of the patent is sent.

Still referring to FIG. 6, collector 116, meter 114 a, and/or meter 114 b may regain synchronization through communication of outbound packets. For example, meter 114 a may be a device on the communication system that has lost synchronization. Meter 114 a may have lost synchronization through a power outage for example. Meter 114 a may regain synchronization by transmitting a synch request packet on the same frequency for a period of time looking for a synch response. In one embodiment, the synch request packet may be a 26 byte, 3.64 ms, packet sent on the same frequency every 400 ms looking for a synch response. The synch request packet may be addressed to a single device. For example, meter 114 a may send a synch request packet to collector 116 (the parent) looking for a synch response. The synch request message may alternatively be a broadcast message. Collector 116 and/or meter 114 b may hear the synch request and respond by transmitting a time synch message to meter 114 a. Collector 116 and/or meter 114 b may transmit a time synch response, such as packet A 602 and/or packet B 604 for example, after receipt of the synch request as described herein.

The transmission type of a synch request packet may depend on whether or not meter 114 a is registered on the communication system. For example, meter 114 a may transmit a request packet to a single device if it is registered on the communication system. If the meter 114 a is not registered on the communication system, the meter 114 a may transmit a broadcast message.

All or portions of the methods and metering devices described above may be embodied in hardware, software, or a combination of both. When embodied in software, the methods and metering devices of the present invention, or certain aspects or portions thereof, may be embodied in the form of program code (i.e., computer executable instructions). This program code may be stored on a computer-readable medium, such as a magnetic, electrical, or optical storage medium, including without limitation, a floppy diskette, CD-ROM, CD-RW, DVD-ROM, DVD-RAM, magnetic tape, flash memory, hard disk drive, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer or server, the machine becomes an apparatus for practicing the invention. A device on which the program code executes, such as meter 114 or collector 116 will generally include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. The program code may be implemented in a high level procedural or object oriented programming language. Alternatively, the program code can be implemented in an assembly or machine language. In any case, the language may be a compiled or interpreted language. When implemented on a general-purpose processor, the program code may combine with the processor to provide a unique apparatus that operates analogously to specific logic circuits.

While methods and apparatus have been described and illustrated with reference to specific embodiments, those skilled in the art will recognize that modifications and variations may be made without departing from the principles described above and set forth in the following claims. For example, although in the embodiments described above, the methods and apparatus are described in the context of a network of metering devices, such as electricity, gas, or water meters, it is understood that the present invention can be implemented in any kind of network. Also, while the exemplary metering system described above is a fixed communication system, the present invention can also be employed in mobile (walk by/drive by) systems. Accordingly, reference should be made to the following claims as describing the scope of the present invention. 

What is claimed is:
 1. In a frequency hopping communication system, in which a transmitter and a receiver hop through a sequence of predetermined communication frequencies, the receiver and the transmitter communicating on each frequency in the sequence for a predetermined period of time before hopping to a next frequency in the sequence, a method for synchronizing the receiver with the transmitter, comprising, at the receiver: for a predetermined amount of time before and a predetermined amount of time after each expected hop from one frequency to a next frequency in the sequence, scanning at least both said one frequency and said next frequency for a carrier signal transmitted by the transmitter; detecting a carrier signal transmitted on said one frequency or said next frequency as a result of said scanning; locking onto the carrier signal; and continuing to hop in sequence with the transmitter.
 2. The method recited in claim 1, wherein in addition to scanning said one frequency and said next frequency for said predetermined amount of time before and said predetermined amount of time after an expected hop, the method further comprises scanning an additional integer number (N) of consecutive frequencies in said sequence immediately before said one frequency and an additional N consecutive frequencies in said sequence immediately after said next frequency, wherein N is not greater than
 4. 3. The method recited in claim 1, wherein the predetermined period of time for which the receiver and transmitter communicate on a given frequency before hopping to a next frequency comprises 400 milliseconds, and wherein the predetermined amount of time before an expected hop from one frequency to the next comprises 200 milliseconds, and wherein the predetermined amount of time after the expected hop from one frequency to the next comprises 200 milliseconds.
 4. The method recited in claim 1, further comprising: periodically receiving from the transmitter a time synchronization message; and synchronizing a clock within the receiver with a clock within the transmitter based on the time synchronization message.
 5. The method recited in claim 4, wherein the receiver receives the time synchronization message from the transmitter about every fifteen minutes.
 6. The method recited in claim 1, further comprising: after the receiver has locked onto the carrier signal, receiving a packet of information from the transmitter, wherein the packet includes a value indicative of a value of a clock of the transmitter; and adjusting a value of a clock within the receiver based on the value in the received packet.
 7. The method recited in claim 1, further comprising: transmitting a synch request message; receiving from the transmitter a time synchronization message in response to the synch request message; and synchronizing a clock within the receiver with a clock within the transmitter based on the time synchronization message.
 8. A metering device comprising: a processor; a transceiver in communication with a transmitter; and memory having stored therein instructions that when executed by the processor cause the metering device to perform the following steps: for a predetermined amount of time before and a predetermined amount of time after an expected hop from one frequency to a next frequency in a sequence of predetermined communication frequencies, scan at least both said one frequency and said next frequency with said transceiver for a carrier signal transmitted by the transmitter; detect a carrier signal transmitted on said one frequency or said next frequency as a result of said scanning; lock onto the carrier signal; and continue to hop in sequence with the transmitter.
 9. The metering device recited in claim 8, wherein in addition to scanning said one frequency and said next frequency for said predetermined amount of time before and said predetermined amount of time after an expected hop, the instructions further cause the metering device to scan an additional integer number (N) of consecutive frequencies in said sequence immediately before said one frequency and an additional N consecutive frequencies in said sequence immediately after said next frequency, wherein N is not greater than
 4. 10. The metering device recited in claim 8, wherein said metering device and said transmitter communicate on a given frequency for a predetermined period of time before hopping to a next frequency.
 11. The metering device recited in claim 8, wherein said predetermined period of time for which the metering device and transmitter communicate on a given frequency before hopping to a next frequency comprises 400 milliseconds, and wherein the predetermined amount of time before an expected hop from one frequency to the next comprises 200 milliseconds, and wherein the predetermined amount of time after the expected hop from one frequency to the next comprises 200 milliseconds.
 12. The metering device recited in claim 8, further comprising a clock, and wherein the instructions further cause the metering device to perform the following steps: after the metering device has locked onto the carrier signal, receive a packet of information from the transmitter, wherein the packet includes a value indicative of a value of a clock of the transmitter; and adjust a value of the clock within the metering device based on the value in the received packet.
 13. The metering device recited in claim 12, wherein the packet of information further comprises a time synchronization message, and wherein said adjusting the value of the clock within the metering device based on the value in the received packet further comprises synchronizing the clock within the metering device with the clock of the transmitter based on the time synchronization message.
 14. The metering device recited in claim 12, wherein the metering device receives the time synchronization message from the transmitter about every fifteen minutes.
 15. A metering system comprising a plurality of metering devices communicating in a frequency hopping communication system, said metering system comprising: a transmitter that transmits a carrier signal on a frequency in a sequence of communication frequencies for a predetermined period of time before hopping to a next frequency in the sequence; a receiver configured to perform the following steps: for a predetermined amount of time before and a predetermined amount of time after an expected hop from one frequency to a next frequency in the sequence of communication frequencies, scanning at least both said one frequency and said next frequency for the carrier signal transmitted by the transmitter; detecting the carrier signal on said one frequency or said next frequency as a result of said scanning; locking onto the carrier signal; and continuing to hop in sequence with the transmitter.
 16. The metering system recited in claim 15, wherein in addition to scanning said one frequency and said next frequency for said predetermined amount of time before and said predetermined amount of time after the expected hop, the receiver is further configured to scan an additional integer number (N) of consecutive frequencies in said sequence immediately before said one frequency and an additional N consecutive frequencies in said sequence immediately after said next frequency, wherein N is not greater than
 4. 17. The metering system recited in claim 15, wherein the predetermined period of time for which the transmitter is transmitting on a given frequency before hopping to a next frequency comprises 400 milliseconds, and wherein the predetermined amount of time before an expected hop from one frequency to the next comprises 200 milliseconds, and wherein the predetermined amount of time after the expected hop from one frequency to the next comprises 200 milliseconds.
 18. The metering system recited in claim 15, wherein the receiver is further configured to perform the following steps: periodically receiving from the transmitter a time synchronization message that comprises at least one of an absolute or relative time; and synchronizing a clock within the receiver with a clock within the transmitter based on the time synchronization message if the time synchronization message comprises an absolute time.
 19. The metering system recited in claim 18, wherein the receiver receives the time synchronization message from the transmitter about every fifteen minutes.
 20. The metering system recited in claim 15, wherein the receiver is further configured to perform the following steps: after the receiver has locked onto the carrier signal, receiving a packet of information from the transmitter, wherein the packet includes a value indicative of a value of a clock of the transmitter; and adjusting a value of a clock within the receiver based on the value in the received packet. 